PET studies on the mechanisms of action of antidepressant and antipsychotic drugs

Abstract: Positron emission tomography (PET) is a non-invasive molecular imaging technique suitable for examination of neurochemical biomarkers in the living brain. Among these applications, PET studies are used to facilitate the development of novel psychotropic drugs. The general aim of this thesis work was to develop and implement novel PET imaging paradigms suitable for research and drug development in psychiatry. The work was carried out in nonhuman primates (NHP) with the intention to prepare for future human applications. The first part of the thesis work was to develop improved PET imaging paradigms sensitive to changes in serotonin (5-HT) concentration. In Study I, the binding of [11C]Cimbi-36, a novel 5-HT2A receptor agonist radioligand, was found to be sensitive to fenfluramine-induced 5-HT release. The 5-HT sensitivity of [11C]Cimbi-36 was comparable to that of [11C]AZ10419369, a 5-HT1B receptor radioligand with established 5-HT sensitivity. In Study II, [11C]AZ10419369 binding was found to be sensitive to pretreatment with 5-HT concentration enhancers (amphetamine, MDMA or 5-HTP) at clinically relevant doses that may be safely applied in human studies. After validation, these PET methodologies were used in Study III to assess the mechanisms of action of vortioxetine, a novel antidepressant. At doses with comparable and clinically relevant occupancy of the 5-HT transporter, vortioxetine induced larger reductions in [11C]AZ10419369 binding than citalopram, and had no significant effect on [11C]Cimbi-36 binding. The results suggest that vortioxetine binds to the 5-HT1B receptor when administered at clinically relevant doses. The second part of the thesis work extended the application of PET imaging from neuroreceptors to intracellularly located enzymes, such as phosphodiesterase 10A (PDE10A). In Study IV, we characterized the binding of [11C]Lu AE92686 to PDE10A in the NHP brain and validated the quantification methods. In Study V, we examined the effect of changes in the concentration of 3',5'-cyclic adenosine monophosphate (cAMP) on [11C]Lu AE92686 binding. Decreasing cAMP concentration by administration of SCH 23390 alone or in combination with R-apomorphine significantly decreased striatal [11C]Lu AE92686 binding. The combination of SCH 23390 and R-apomorphine also significantly increased binding in substantia nigra. The decrease in striatal [11C]Lu AE92686 binding may reflect a decrease in PDE10A affinity induced by cAMP depletion. The effect of changes in cAMP concentration on PDE10A binding should thus be considered when [11C]Lu AE92686 is used in human studies. In conclusion, the current thesis work has advanced the PET imaging paradigms for examining changes in 5-HT concentration. The validation of [11C]Lu AE92686 as a PDE10A PET radioligand and understanding the modulatory role of cAMP on [11C]Lu AE92686 binding will facilitate the application of this PDE10A radioligand in future studies. The developed PET methodologies were applied to evaluate the mechanisms of action of psychotropic drugs and can be translated into future human studies.